1. Field of the Invention
This application relates to perimeter intrusion detection systems of the type using the disturbance of an electromagnetic field around a cable to detect intruders. In particular, the invention relates to systems which establish the approximate location of an intruder or target detected by the system. The cables used in the systems of this invention are divided into sections or blocks and the system indicates in which section the intrusion has occurred. This is known as "block ranging".
2. Background Information
As an example of a perimeter intrusion detection system reference may be made to U.S. Pat. No. 4,091,367 (Canadian Patent No. 1,014,145) which teaches the use of a pair of leaky coaxial cables buried in the ground and extending around such a perimeter. The leaky cables disclosed in this patent have a series of apertures to permit coupling outside the cable. Apertures are not an essential feature of a leaky cable and other means may be used to provide the externally coupled field, e.g. diffusion through a thinner shield. A transmitter supplies r.f. energy to one of the leaky cables and a receiver is connected to the other. The field set up by the transmitting cable links with the receiving cable. The resulting signal at the receiver has a fixed component, known as "profile" plus a variable component formed by any change in the received energy resulting from the presence of a target. A signal processor at the receiver separates the profile and target signals, compares the target signal with a threshold and activates an alarm if the threshold is exceeded.
The magnitude of the target signal varies with position along the cable; the coupled energy in the vicinity of targets further away from the transmitter-receiver location being less due to cable attenuation. One manner of compensating for this unwanted variation in target signal is to grade the cable, by changing aperture size or distribution, so that more energy is coupled at locations remote from the transmitter and the target signal is then substantially independent of target location. Another manner of reducing variations in sensitivity with target location is to connect the receiver to the end of its cable which is remote from the transmitter so that the signal path length for any target position is constant. Sensing with this arrangement is termed co-directional sensing and sensing as originally described with the transmitter and receiver adjacent is termed contra-directional sensing.
By use of the timing of pulse return signals with contra-directional sensing as in conventional radar, the target location along the length of the cable pair may be determined. This can provide accuracy of the order of several meters on a cable length of about 800 m. if pulse interpolation or wide bandwidth is employed. Typically, however, the display will be divided in blocks of length 33-100 m. and the extreme accuracy is not required. Each block may have a different threshold to compensate for variations in sensitivity.
If a continuous wave r.f. signal is used then there is no target timing information on the received signal and only the presence of a target somewhere along the cable is determined. Cable length is typically limited to about 150 m. because of difficulty in compensating for sensitivity variations along the cable. Nevertheless, c.w. systems are attractive since the absence of pulse circuitry and r.f. switching leads to reduced cost. British Patent Specification No. 2,120,823 published Dec. 7, 1983, suggests dividing the perimeter into sectors each driven by a separate transmitter and receiver forming a remote terminal. A control unit is connected at one end of the cables and polls each of the remote terminals in turn to determine if any of the associated sectors have been intruded. Thus, block ranging is achieved by providing a separate cable and separate processing electronics for each block to be covered together with the control unit at one end of the cables. This results in increased cost and also requires some of the processing electronics to be operated remotely from the central control in a harsh external environment.
A system proposed in British Patent Specification No. 83 24686, in the name of Martin, uses switch boxes distributed periodically along a single leaky coaxial cable. A signal transmitted by a cooperating vehicle whose position is being tracked is modified by adding a marker pulse at each switch box. The base station determines vehicle location by counting the number of marker pulses. Clearly this system is not applicable to intruders.
The present invention involves the concept of switching alternative cable sections into active use, typically one at any time. A particular form of switching a cable section into and out of active use is disclosed in U.S. Pat. No. 4,213,123 to Poirier. Poirier provides a gate or "entry portal" by replacing a short section of the leaky cable used in a detection system by a conventional non-leaky cable during the period a vehicle will be entering so that there is no transmitted field at the gate. The patent is not concerned with the problem of providing block ranging of a target which is the concern of the present application.
In the previously discussed system of U.S. Pat. No. 4,091,367 the cables are buried beneath the surface of the ground in order to maintain the system covert. This has the added advantage that interaction of differing modes of electromagnetic propagation are largely avoided, since the ground is lossy. It is known that for leaky cables in air, these interacting modes can produce "mode cancellation", or standing waves which cause the sensitivity to targets to be non-uniform with distances along the cable length.
An alternative system is taught in applicant's copending application Serial No. 116,585, filed Nov. 4, 1987 using waves propagating along the outer conductor of a coaxial cable. Such waves are similar to those produced from the apertures of a leaky coaxial cable but without the disadvantage of interaction between them mentioned above. The use of such waves for communication purposes is disclosed in U.S. Pat. No. 3,829,767 issued Aug. 13, 1974 to Delogne. As an alternative to leaky coaxial cables Delogne teaches the use of ordinary coaxial cables with passive elements called mode converters inserted at intervals. These elements inserted in a normal non-leaky coaxial line couple some of the energy between the cable interior and exterior. The spacing distance is dependent on the attenuation of modes of propagation exterior to the cable, which vary typically from about 1 .dB/100' for cables in air to 1 .dB/foot for buried cables. As an illustration, spacings in air of about 300 ft. are possible.
As taught by Delogne two types of waves may be generated outside the coaxial cable, one which propagates radially away from the source and the other which is guided along the outer surface of the cable. It is this second type of wave which is used for obstacle or intrusion detection. Some embodiments of the present invention use a regular coaxial cable with spaced coupled wave devices, as opposed to a leaky coaxial cable. The function of these devices, hereinafter called CWD's, is to transfer r.f. energy efficiently from a transmission mode within the cable to a guided mode along the exterior of the cable with minimal undesirable reflections or transmission losses.
The block ranging system of this invention can be utilized both with leaky coaxial cables and with conventional cables having r.f. energy transmitted in a guided mode on the exterior of the cable. It requires only c.w. signals and does not involve the duplication of transceiver units. No high speed electronic switching and timing of r.f. pulses is required and relatively short blocks may be monitored.